The present invention relates to furnaces and particularly to improved apparatus for regulating air flow through a port introducing combustion air into the firebox of a furnace and including apparatus for automatically cleaning the air port.
Wood pulp for papermaking is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips and the cooking liquor, called "white liquor," are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor," containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are reclaimed. Without reclamation and reuse of the cooking chemicals, the cost of the papermaking process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about 65 percent solids, which solution is then sprayed into the firebox of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the firebox, and the heat produced dries and melts the spent cooking chemicals as they fall to the floor of the firebox, where they build a mound of material called a char bed. The char bed is further heated to further liquefy the chemicals into a molten smelt that flows out of the furnace through a smelt spout to a collection tank. Concurrently, combustion heat is employed to generate steam in a water wall of the boiler for use as process steam and for generating electricity.
The combustion process requires the introduction of large volumes of air into the firebox, air comprising about 80 percent of the material entering the furnace. The combustion air is distributed by means of wind boxes or ducts disposed at several levels in surrounding relation to the firebox and outside the walls of the furnace. The air is forced into the firebox from the wind boxes through a plurality of passages or air ports in the walls of the furnace, viz.: primary, secondary and tertiary air ports. The primary air ports, through which about 40 to 50 percent of the air enters the furnace, are disposed on the side walls of the firebox near the bottom of the furnace and close to the char bed. The air supplied to the primary air ports is at a comparatively low pressure in order to promote a reducing atmosphere in the burning mass of char. The secondary air ports, which are fewer in number than the primary air ports and through which about 35 percent of the air enters the furnace, are disposed around the walls of the firebox, higher than the primary air ports, and usually below the level of the entry conduits through which the black liquor is sprayed into the firebox. Air supplied through the secondary air ports is at a slightly higher pressure in order to promote burning of combustible gasses rising from the glowing mass of the char bed. While the primary air ports provide a relatively large volume of air with considerable turbulence for maintaining a fireball in the char bed, the secondary air ports are intended to provide a finer control and distribution of air above the char bed and distribute the air evenly in the black liquor spray to support the combustion thereof. Air is supplied through the tertiary air ports at a still higher pressure to promote combustion of gases rising through the firebox, the tertiary air ports being higher on the wall of the furnace than the secondary air ports.
The black liquor sprayed into the firebox, having a consistency like warm 60 weight oil, swirls, burns and falls toward the bottom of the firebox in the form of combustion products comprising char material and smelt. The smelt and char material contact and flow down the outer walls of the firebox and, cooled by the inflowing air, form excrescent deposits around edges of the air ports, particularly along the top edges of the secondary air ports where the excrescent material builds up and outward under influence of air rushing through the secondary air port. Such buildup of char material can block air flow through a port by as much as ten percent. Therefore, furnace operation tends to be inefficient and unpredictable with an attendant decrease in the amount of chemicals that can be recovered, a decrease in the amount of steam produced per unit of fuel, and increased emission of noxious gases such as hydrogen sulfide, carbon monoxide and sulfur dioxide. In accordance with customary practice, the char buildup is periodically removed either by manually inserting a cleaning rod into the air ports successively around the boiler or by actuating mechanized cleaning apparatus mounted in the air ports. With the passage of time, and particularly when cleaning is effected by manual rodding of the air ports, gradual buildup of char material intermittently around the furnace can cause changes in the volume of combustion air, as well as changes in air distribution, velocity and pressure
The volume and distribution of combustion air supplied to the furnace will also vary depending on the load of the furnace and the moisture content of the liquor being reduced. The distribution and volume of air entering a furnace is conveniently adjusted by regulating means such as dampers provided in supply conduits of the wind boxes. Dampers may also be provided at various locations in the wind boxes, and individual air ports may furthermore be provided with a damper, thus making possible a selective distribution of air within each wind box, or in each wind-box passage or each air port, respectively, thereby maintaining the desired air supply in all parts of the furnace.
Separate apparatus for cleaning openings in a recovery furnace are known. See, for example, my U.S. Pat. No. 4,423,533 entitled FURNACE AIR PORT CLEANER. Apparatus combining the function of airport cleaning and air-flow damping are also known; however, such dual purpose apparatus have been found to have certain disadvantages in that either the air damping or the cleaning is unsatisfactory. Cleaning apparatus designed to be effective for that purpose, when used as a damper, may be subjected to excessive heat from the furnace and deteriorate rapidly. My copending application Ser. No. 829,712, now U.S. Pat. No. 4,748,004, discloses a cleaning head for use in secondary air ports that may be employed also as a damper to control the flow of combustion air through the air port. For this purpose, the cage-like structure of the cleaning head is enclosed, and the mounting frame is partially or completely enclosed or walled in, so that, for a given position of the cleaning head, air flow tends to be closed off. The position of the cleaning head may be varied to accomplish cleaning or to effect a different air flow. It has been found that a cleaning element utilized as a damper partially or fully blocking an air port, i.e., extending into the air port, often is subjected to excessive heat from the furnace. It is thus desirable to employ a separate damper that is disposed near, but spaced apart some distance inside the wind box from the air-port opening.
On the other hand, a device designed to function effectively as a damper, when extended into the air port for cleaning the same, often proves to be ineffective for that purpose because the shape of the damper is not conducive to cleaning the opening. Accordingly, separate air damping and air-port cleaning apparatus are desirable over dual purpose devices because more uniform and stable air flow is maintained through the air ports, resulting in more efficient operation of the furnace.
A damper that controls the air flow to a particular air port ordinarily is located near the air port, and consequently may interfere with the operation of separate automatic cleaning apparatus installed in the air port. If unlimited space were available, a damper could be installed upstream of the cleaning apparatus in the passage supplying air to the air port; however, space adjacent to an air port for installing such apparatus is often limited and constructing such additional space is costly. Further, when a damper is retracted or moved out of the way of cleaning apparatus, it is often necessary to position the damper such that air flow through the passage to the air port is blocked, which is undesirable because without positive air pressure outside the air port, effluent from the furnace could enter the air duct and foul or damage the mechanisms.
It is accordingly a primary object of the present invention to provide improved apparatus for regulating the flow of combustion air in a chemical recovery furnace.
It is another object of the present invention to provide improved air regulating apparatus installable in an air port of a chemical recovery furnace and including air-port cleaning apparatus that operates cooperatively with a damper mechanism.
Another object of the present invention is to provide improved air regulating apparatus including air port cleaning apparatus for increasing the operational stability of a black liquor recovery boiler.
It is a further object of the present invention to provide improved air regulating apparatus operating in concert with air port cleaning apparatus for enhancing the efficiency of chemical recovery, increasing steam production, and reducing sulfur dioxide and carbon monoxide emissions from a black liquor recovery boiler.